1. Field of the Invention
This invention relates to a backlight, of a display device, having a discharge tube emitting light during discharge in a lean gas and to a method of manufacturing a backlight, and a display device.
2. Description of the Related Art
A backlight, of a display device such as a liquid crystal display device, uses a light source device comprising one or a plurality of discharge tubes and a reflector. The discharge tube is a cold cathode tube, in which mercury and argon (Ar) gas or neon (Ne) gas are sealed and a fluorescent material is coated on the tube wall. Mercury gas generates ultraviolet light during discharge, and the ultraviolet light impinges against the fluorescent material and generates visible light.
The backlight of most of liquid crystal display devices includes a light guide plate. In one example, two light source devices are disposed on opposite sides of the light guide plate in such a manner as to face each other. Each light source device comprises two discharge tubes and a reflector. In this arrangement, two discharge tubes having a diameter of several millimeters are disposed in a narrow region of not greater than 10 mm. Therefore, the ambient temperature around the discharge tubes often reaches 70° C. or more.
The light intensity-temperature characteristics of a discharge tube have a tendency such that the light intensity drops in a high temperature region for the following reason. First, considering the amount of ultraviolet light generated by the mercury gas, the amount is substantially proportional to the mercury gas concentration and the current. On the other hand, mercury gas has the property of absorbing ultraviolet light, and the absorption factor changes exponentially, with the product of the mercury gas concentration, over the distance the ultraviolet light must travel (the transmission factor changes as the concentration and the distance of transmission become greater). The ultraviolet light is converted into visible light by the fluorescent material coated on the tube wall. The product of the diameter of the discharge tube and the concentration of the mercury gas determines the probability of the incidence of one UV photon on the fluorescent material. From the explanation given above, the light intensity I of the visible light can be expressed as follows, wherein “d” is the tube diameter, “n” is the mercury gas concentration (a function of the temperature of discharge tube) and “J” is the current.I˜k(J×n)×exp(−b×n×d)  (1)                (where k and b are proportional constants.)        
Equation (1) shows that I is likely to assume the maximum value for a predetermined mercury gas concentration n, and when the mercury gas concentration becomes higher than this predetermined concentration n, the intensity of the visible light drops. The mercury gas concentration changes exponentially with the temperature of the mercury gas and, therefore, the brightness becomes lower in a high temperature region as the tube temperature becomes higher. Since the tube temperature becomes higher as the current increases, the intensity of the visible ray decreases when the current is increased at a predetermined ambient temperature. These decreases cause a problem when the brightness of the backlight is to be increased.
Japanese Unexamined Patent Publication (Kokai) No. 5-225819 discloses control of the brightness of a discharge tube by fitting a cooling metal device to the discharge tube to cool the whole discharge tube.
Japanese Unexamined Patent Publication (Kokai) No. 60-16813-discloses a light source for a copying machine. This light source comprises a fluorescent lamp, a lamp heater encompassing the fluorescent lamp and a heat pump disposed at a notched portion of the lamp heater. The heat pump absorbs heat from the tube wall of the fluorescent lamp and controls the light intensity of the fluorescent lamp by controlling the mercury vapor pressure in the fluorescent lamp tube.
However, it is difficult to apply the technology of fitting the cooling metal device for the discharge tube to the light source device of the backlight of the display device. That is, as the tube diameter is small in the backlight of the display device and a reflector exists around the discharge tube, a large cooling metal device cannot be fitted to the discharge tube. Also, leakage of current through the cooling metal device becomes large and, as the backlight of the display device has smaller power consumption than the fluorescent lamp for ordinary illumination, the discharge tube is likely to be excessively cooled. For these reasons, this technology is not practical.
The light source device having a lamp heater that encompasses the fluorescent lamp cannot be used for the backlight of the liquid crystal display device.
Japanese Unexamined Patent Publication (Kokai) No. 2000-323099 discloses a method of fabricating a fluorescent lamp in which liquid mercury gathers at the central portion of a fluorescent lamp to prevent the central portion of the fluorescent lamp from becoming dark for several seconds of its use after the lamp is left standing for a long time in a cold environment, because liquid mercy gathers at the ends of the fluorescent lamp. This prior art discloses that when the fluorescent lamp is cooled and the temperature of its central portion becomes lower, by about 10° C., than the temperature at the ends thereof, liquid mercury gathers at the central portion of the fluorescent lamp. In practice, however, almost all the liquid mercury in the fluorescent lamp does not gather at one position in the fluorescent lamp.
A study of small diameter discharge tubes, for use in the backlight, conducted by the inventors has revealed that almost all the liquid mercury does not gather at a position remote from the end portions of a discharge tube having the inner diameter of 5 mm or below even when the discharge tube is assembled into a backlight. When electric current is applied to activate the discharge tube, liquid mercury generally gathers at one of the end portions because of the asymmetry of the waveform.
Even when the waveform is symmetric, liquid mercury that is arbitrarily distributed in the discharge tube does not gather easily at one position in the discharge tube of a backlight using a thin glass tube of 5 mm or less, because the tube is thin and contains an amount of liquid mercury considerably greater than the amount of the gaseous mercury necessary for discharge. According to experiments conducted by the inventors, a time of 200 to 1,000 hours is necessary to collect the liquid mercury at one position in the discharge tube. During the process in which liquid mercury gathers at one position, degradation of the fluorescent lamp proceeds and the brightness drops.
According to one aspect, the present invention provides a backlight capable of improving brightness by forming a most-cooled portion at a predetermined position in a discharge tube. However, it has been found that, in such a backlight, the desired improvement of brightness cannot be achieved unless liquid mercury is collected at the predetermined position. In another aspect, therefore, the present invention provides a backlight in which liquid mercury is collected at a predetermined position of a discharge tube, and this predetermined position is the most cooled portion.
The display device having the backlight containing the discharge tube involves another problem in that, even when the current supplied to the discharge tube is increased, the brightness does not increase much.
In the case of the liquid crystal display device using linearly polarized light, only a half of light of a non-polarized light source is utilized, hence, the utilization efficiency of light is low. A proposal is therefore made to dispose a polarization separating element in the backlight of the display device to improve utilization efficiency of light. The polarization separating element comprises a reflection type polarization plate (polarization separating sheet) sandwiched between a light guide plate and a liquid crystal panel. The reflection type polarization plate allows first linearly polarized light of the ray of light traveling from the light guide plate to the reflection type polarization plate to transmit therethrough but reflects second linearly polarized light having a plane of polarization crossing orthogonally the plane of polarization of first linearly polarized light. The plane of polarization of second linearly polarized light, that is again made incident to the light guide plate, is converted by means for converting the second linearly polarized light to first linearly polarized light. Therefore, the second linearly polarized light travels again as first linearly polarized light from the light guide plate to the reflection type polarization plate and is transmitted through the latter. In this way, the utilization efficiency of the light can be improved, and a display device having higher brightness can be achieved.
In the conventional backlight, a diffusion reflection plate is disposed as first means for converting second linearly polarized light to first linearly polarized light below the light guide plate. Second linearly polarized light that is reflected by the reflection type polarization plate and is again made incident to the light guide plate is scattered and reflected by the diffusion reflection plate to non-polarized light. As the non-polarized light is thus made incident to the reflection type polarization plate, at least a part of the second linearly polarized light can be utilized, and utilization efficiency of light can be improved in comparison with the case where second linearly polarized light is not at all utilized. However, a part of second linearly polarized light is scattered and reflected by the diffusion reflection plate, is scattered in the periphery of the light guide plate without traveling from the light guide plate to the reflection type polarization plate, and is absorbed by the light source and the casing. Therefore, the utilization efficiency of light is still limited.
In another example, a λ/4 plate is disposed as second means for converting second linearly polarized light into first linearly polarized light below the reflection type polarization plate, and an isotropic metal mirror is disposed below the light guide plate. Second linearly polarized light reflected by the reflection type polarization plate passes through the λ/4 plate, changes to left (right) circularly polarized light, is reflected by the isotropic metal mirror, changes to right (left) circularly polarized light, again passes through the λ/4 plate and changes to first linearly polarized light. Since first linearly polarized light transmits through the reflection type polarization plate, utilization efficiency of light is improved. In this case, however, the isotropic metal mirror absorbs the rays of light, so that utilization efficiency of light is limited, too.
In addition, backlights of the liquid crystal display device include a “side light type” backlights and a “direct illumination type” backlights. The side light type backlight includes a light guide plate and a light source disposed on the side of the light guide plate, and has the advantage that a thin liquid crystal display device can be provided. The direct illumination type backlight includes a light source radiating the ray of light to the liquid crystal display device, and has the advantage that a high brightness of the liquid crystal display device can be accomplished. However, the direct illumination type backlight cannot easily provide a liquid crystal display device that is thin and has low power consumption, and involves the problem that non-uniformity of brightness is likely to occur. Therefore, the side light type backlight has gained a wider application in recent years.
Besides the light guide plate and the light source described above, the side light type backlight includes a reflection mirror (reflection film) disposed below the light guide plate (on the far side from the liquid crystal panel) and an optical sheet disposed above the light guide plate (on the near side to the liquid crystal panel). The light outgoing from the light source is made incident to the light guide plate. While the light propagates in the light guide plate, the reflection mirror reflects a part of the light, and the reflected light goes from the light guide plate and is made incident to the liquid crystal panel through the optical sheet.
The optical sheet regulates the brightness distribution of light outgoing from the light guide plate. In other words, since the light going from the light guide plate contains a large quantity of the light that describes a large angle to the normal to the light guide plate, the optical sheet mainly converts the light describing a large angle to the normal to the light guide plate into the light describing a small angle to the normal to the light guide plate.
FIG. 123 illustrates a prism sheet as one of the optical sheets. The prism sheet 1 is a transparent sheet having a large number of prisms 2 formed thereon. The light X made incident to the prism sheet 1 with a large angle to the normal of the prism sheet 1 goes from the prism sheet 1 with a small angle to the normal of the prism sheet 1. Therefore, an observer of a liquid crystal panel behind the prism sheet 1 can easily view the liquid crystal panel from the front surface. On the other hand, the prism 2 reflects the light Y that is made incident to the prism sheet 1 along the normal of the prism sheet 1. Therefore, this ray Y returns.
Incidentally, Japanese Unexamined Patent Publications (Kokai) No. 6-273762, No. 8-146207, No. 9-15404, No. 10-97199, No. 10-246805 and No. 2000-56105 disclose examples of the prism sheets and the scatter sheets. Japanese Unexamined Patent Publication (Kokai) No. 11-329042 describes an example of the planar light source.
As described above, the prism sheet 1 has fine prisms 2 formed on the sheet surface in order to allow the light, that is made incident to the prism sheet 1 at an angle in an oblique direction, to enter in the front surface direction or at an angle approximate to the front surface. In this prism sheet 1, the quantity of the outgoing light within the outgoing angle range determined by the shape of the prisms 2 is large, and the quantity of the outgoing light outside the outgoing angle range drops drastically. In other words, the brightness distribution of the light made incident to the liquid crystal panel changes drastically. Therefore, the conventional technology combines the prism sheet 1 with the scatter sheet containing scattering material particles to achieve a wide brightness distribution such that the quantity of the light at an angle in the normal direction becomes maximum and becomes progressively smaller as the angle becomes greater, from the normal direction. The light Y made incident to the prism sheet 1 along the normal to the prism sheet 1 is returned to the light guide plate side, and utilization efficiency of light drops. Further, the light X that is inclined to one side leaves from near one of the ends of the light guide plate, and the light Z inclined to the other side outgoes from near the other end of light guide plate. This tendency remains even after the light is transmitted through the prism sheet.
The production cost of the prism sheet 1 is high because the fine prisms 2 must be fabricated accurately in the prism sheet 1. The prism sheet 1 itself does not have a light absorbing property, but the ray of light returned towards the light guide plate is absorbed by the reflecting mirror, the light source, the casing frame, etc, with a drop in utilization efficiency. When the prism sheet and the scatter sheet containing the scattering material particles are used in combination, the production cost increases due to an increase in the cost of the sheet itself and an increase in the cost and the number of assembly steps. Further, a problem of a drop in the yield occurs because dust appears between the prism sheet and the scatter sheet.
As the thickness of the liquid crystal display device is required to be smaller and smaller, the light guide plate becomes smaller and smaller, too. When the thickness of the light guide plate is reduced, however, the quantity of incoming light from the sides of the light guide plate becomes small. Therefore, a reduction of thickness of the side edge type backlight is limited.
To cope with a reduction in the thickness of the light guide plate, a method has been proposed to input the rays of light from the upper or lower surface of the light guide plate (e.g. Japanese Unexamined Patent Publication (Kokai) No. 11-329042). This proposal employs a structure in which a part of a flexible film is curved to form a cylindrical portion and the light source is positioned in this cylindrical portion so as to guide the light received from the cylindrical portion to other portions of the flexible film. However, according to this construction, a part of the light irradiated from the light source onto a part of the cylindrical portion is guided to other portions, but another part of the light travels to the other side of the flexible film. Furthermore, the scatter-reflection layer reflects still another part of the light irradiated from the light source to the cylindrical portion, and the light returns to the light source lowering, thereby, the utilization efficiency of light.